The present invention relates to a light-emitting-diode array and a fabrication method thereof, more particularly to an inexpensive method of fabricating a high-density light-emitting-diode array.
A light-emitting diode (also referred to as an LED) is basically a pn junction. It is known technology to fabricate a light emitting-diode array by diffusing a p-type impurity such as zinc through a diffusion mask having multiple diffusion windows into an n-type semiconducting substrate such as n-type gallium arsenide (GaAs). In conventional light-emitting diode arrays, the underside of the substrate is coated with a metal film that serves as a common cathode electrode, and an individual metal anode electrode is provided for each light-emitting diode on the upper side. The anode electrode pattern for each light-emitting diode comprises a comparatively large bonding pad, formed on the diffusion mask, or on an inter-layer insulating film, and a narrower line connecting the bonding pad to the light-emitting diode. The bonding pads are coupled by wire bonding to a separate integrated circuit (IC) that drives the light-emitting-diode array. Arrays of this type with a single row of light-emitting diodes are used as light sources in electrophotographic printers.
With the electrode structure described above, however, the density of the light-emitting diodes is limited by the density with which the bonding pads can be laid out and the bonding wires attached. Even by placing the bonding pads on alternate sides of the row of light-emitting diodes, it is difficult to achieve densities as high as twelve hundred light-emitting diodes per inch (1200 dpi), or higher, which are desired densities for high-quality printing. Moreover, even if a light-emitting-diode array of this density could be fabricated, connecting the array to its driver ICs by wire bonding would present a difficult and perhaps insurmountable challenge.
To overcome this problem, Japanese Kokai Patent Publication No. 152873/1987 discloses a matrix driving scheme in which the light-emitting diodes are divided into groups, each group being formed in a separate n-type well in a p-type substrate. Each n-type well is coupled through an npn bipolar transistor to the common cathode electrode of the array. This arrangement reduces the number of bonding pads, but the bipolar transistors add considerably to the size, complexity, and fabrication cost of the array.
Japanese Kokai Patent Publication No. 177478/1988 discloses a matrix driving scheme in which each light-emitting diode is disposed in an individual mesa of semiconductor material that is electrically isolated from the substrate. This scheme causes planarization problems, which can lead to electrical discontinuities in electrode lines.
Japanese Kokai Utility Patent Publication No. 170142/1988 discloses a matrix scheme in which the light-emitting-diode array is divided into sections that are electrically isolated from one another by intervening layers of a dielectric material. This type of dielectric isolation requires extra fabrication process steps, and is not easily achieved at a low cost.
It is accordingly an object of the present invention to provide a low-cost matrix-driven light-emitting-diode array.
Another object of the invention is to provide a low-cost fabrication method for a matrix-driven light-emitting-diode array.
Still another object is to reduce the size of a light-emitting-diode array.
Yet another object is to increase the number and density of the light-emitting diodes in a light-emitting-diode array.
A further object is to assure electrical continuity in the electrode lines of a matrix-driven light-emitting-diode array.
A still further object is to assure uniform light-emission characteristics of the light-emitting diodes in a matrix-driven light-emitting-diode array.
The invented light-emitting-diode array is fabricated on a substrate having a lower layer of an insulating or semi-insulating material and an upper layer of an n-type or p-type semiconducting material. The upper layer is divided into blocks by isolation channels that extend from the upper surface of the upper layer entirely through the upper layer. The blocks are electrically isolated from one another by these isolation channels. Each block has a block electrode that makes electrical contact with the upper layer in the block.
A row of light-emitting diodes is formed by selective diffusion of an impurity into the upper layer. Each block of the upper layer includes a plurality of these light-emitting diodes. The row of light-emitting diodes is paralleled by a plurality of shared lines, each of which is electrically coupled to a plurality of light-emitting diodes disposed in different blocks. Each light emitting diode is electrically coupled to just one of the shared lines.
The isolation channels may have a rectangular cross-sectional shape, or a trapezoidal cross-sectional shape that is widest at the top of the upper layer. Alternatively, the isolation channels may have a rectangular cross-sectional shape where they pass through the row of light-emitting diodes, and a trapezoidal cross-sectional shape where crossed by the shared lines. The isolation channels may be filled with an insulating material, for planarization and for enhanced electrical isolation.
The shared lines are coupled to the light-emitting diodes by individual lines which are separated from the shared lines by an inter-layer insulating film. If the shared lines are formed below this inter-layer insulating film, the shared lines are preferably plated to prevent oxidation and assure good electrical contact with the individual lines. If the individual lines are formed below this inter-layer insulating film, the individual lines are similarly plated.
The block electrodes and individual lines are preferably sintered to assure good electrical contact with the upper layer of the substrate and with the light-emitting diodes. The block electrodes are coupled by block lines to block bonding pads. The block lines and block bonding pads are preferably formed after the sintering of the block electrodes and individual lines.
Each shared line is coupled to at least one shared-line bonding pad. The shared-line bonding pads and block bonding pads are preferably aligned in a straight or zig-zag row on one side of the row of light-emitting diodes. If necessary, however, the shared-line bonding pads and block bonding pads may be disposed on opposite sides of the row of light-emitting diodes.
The invented light-emitting-diode array can be fabricated at a low cost because the isolation channels can be formed by standard photolithography and etching. The size of the array can be reduced by aligning all bonding pads in a single row. The number of light-emitting diodes can be increased, while maintaining uniform light-emitting characteristics, by providing more than one bonding pad per shared line. The density of the light-emitting diodes can be increased, as compared with conventional non-matrix arrays, because it is not necessary to provide a separate bonding pad for each light-emitting diode. Electrical continuity of the shared lines can be assured by appropriate design of the cross-sectional shape of the isolation channels where the shared lines cross these channels, or by filling in the isolation channels.